class Benz:pass
class BMW:pass
class BYD:pass

class CarFactory:
    __cars = {'benz':Benz, 'bmw':BMW, 'byd':BYD}
    __instance = None

    def __new__(cls, *args, **kwargs):
        if cls.__instance == None:
            cls.__instance = object.__new__(cls)
        return cls.__instance
    
    @classmethod
    def get_car(cls, brand):
        c = cls.__cars.get(brand, BYD)
        return c()